A phosphate based glass which possesses a narrow spectral bandwidth is excellent in the laser oscillation characteristic, so that in its use as laser glass especially high repetitive and high output oscillation is strongly desired. However, in such cases, heat from a flash lamp used for the excitation creates a temperature distribution within the glass, causing thermal stress therein. The phosphate based glass, e.g., phosphate or silicophosphate glass, has a low bending strength as compared with silicate glass and it is easily broken due to the thermal stress so that it is not suitable for use in high power laser oscillation.
In order to minimize the thermal stress air cooling or water cooling is usually performed, but it is not so effective because the glass itself has low thermal conductivity. Further, since the phosphate based glass has insufficient resistance to water and moisture, when air cooling or water cooling is performed, staining or dimming often occurs on the surface, resulting in the deterioration of breaking strength and oscillation performance.
In general, the thermal shock resistance R is represented by the following equation: EQU R=(1-.mu.).kappa.S/.alpha.E
wherein E is a Young's modulus, .mu. is a Poisson ratio, .alpha. is a coefficient thermal of expansion, .kappa. is a thermal conductivity, and S is a rupture (breaking) strength.
Since the factors E, .mu., .alpha. and .kappa. are constants for a given substance, in order to improve the thermal shock resistance it is necessary to increase the breaking strength S by any means.
Heretofore, in order to increase the breaking strength of glass there has been carried out a process wherein a compressive stress layer is formed on the surface of the glass by ion exchange using molten salt. However, this process can be applied only to specifically formulated glass compositions and cannot be applied to other glass compositions such as a phosphate based laser glass composition.
A process for increasing the breaking strength of a phosphate laser glass is described in U.S. Pat. No. 4,547,402, wherein the surface of the glass is coated with a sol solution which is prepared by hydrolyzing a metal alcoholate dissolved in an organic solvent and partially polycondensing, and the coated layer is then heat-treated to form a glassy coating on the surface of the laser glass. According to this process, the glassy coating formed eliminates, to some extent, microcracks present on the surface of the laser glass so that the thermal shock resistance can be increased. However, the microcracks cannot be completely eliminated by this method, and moreover the glassy coating does not provide satisfactory water resistance and breaking strength because of insufficient strength of the glassy coating itself.